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USMLE Renal > Physiology - Clearance, GFR, Calculations > Flashcards

Physiology - Clearance, GFR, Calculations Flashcards

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USMLE® Step 1 flashcards
1
Q

How much water is found intracellularly vs extracellularly?

A
  • 1/3 extracellular

- 2/3 intracellular

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2
Q

What percentage of water is interstitial vs plasma?

A
  • 1/4 Plasma

- 3/4 interstitial

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3
Q

What can inulin be useful for?

A

Determining GFR

- As it is neither secreted or resorbed

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4
Q

What space does inulin remain in?

A

Extracellular

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5
Q

What substance stays exclusively in the plasma

A

Radiolabeled Albumin

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6
Q

What is the normal value of plasma osmolarity?

A

~ 300mosm/kg

Equilibrium between cells and extracellular fluid

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7
Q

What kind of fluid causes an increase in both extracellular and intracellular fluid?

A

Hypotonic fluid

- E.g 5% Dextrose

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8
Q

What are the effects of Mannitol on fluid volumes?

A
  • Raises plasma osmolarity
  • Remains in the vascular system
  • Decreases ICF
  • Increases ECF
  • Reduces volume in interstitial space
  • Draws fluid out of brain cells/tissue into plasma
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9
Q

What is the effective circulating volume?

A
  • Extracellular fluid contained in arterial system
  • Maintains tissue perfusion (how well tissues are being perfused essentially)
  • Not necessarily correlated with total body water
  • Modified by: Volume, CO, Vascular resistance
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10
Q

What will a decrease in TPR have on the effective circulating volume?

A

Decrease effective circulating volume

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11
Q

What 2 main conditions cause a decrease in effective circulating volume but an increase in total body water?

A
  • Heart Failure

- Cirrhosis

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12
Q

What 2 main systems are activated by a low effective circulating volume?

A
  • Sympathetic NS

- RAAS

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13
Q

How is the GFR determined?

A

Determined from plasma, urine measurements

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14
Q

What is the Renal Blood Flow/ Plasma Flow?

A

How much blood enters kidney

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15
Q

What is the Filtration Fraction?

A

GFR/RPF

  • Can be used to determine how healthy kidneys are
  • Of all substance X entering kidney, what % gets filtered
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16
Q

What are the different methods of obtaining GFR?

A

Theoretical determination
- Need to know pressures in capillary, Bowman’s capsule

Clinical determination
- Need to know plasma concentration solutes, urine flow

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17
Q

What is the equation to calculate plasma volume?

A

TBV x (1-Hct)

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18
Q

What is a normal Filtration Fraction?

A

~ 20%

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19
Q

What is the effect of constriction of the efferent arteriole on GFR and RPF?

A
  • GFR will increase

- Renal Plasma Flow will decrease

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20
Q

What is the effect of constriction of the afferent arteriole on the GFR and RPF?

A
  • GFR decrease

- RPF decrease

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21
Q

What can dilate afferent arterioles?

A

Prostaglandins (NSAIDs = constrict)

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22
Q

What can constrict efferent arterioles?

A

Angiotensin II

thus ACEi will dilate efferent

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23
Q

Why would multiple myeloma decrease GFR?

A

Increases proteins in blood

  • Oncotic pressure in glomerulus increases
  • GFR decreases
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24
Q

How can an obstructed ureter cause decreased GFR?

A
  • Causes an increases in Hydrostatic pressure in Bowmans capsule
  • Decreased GFR
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25
Q

What are the 2 mechanisms by which the kidneys autoregulate?

A
  • Myogenic mechanism
  • Tubuloglomerular feedback
    Needed to maintain Renal blood flow and GFR to normal levels as BP fluctuates throughout the day
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26
Q

Describe the myogenic mechanism

A

Afferent arteriole constricts in response to high pressure

  • Responds to changes in stretch
  • Maintans normal GFR/RPF
27
Q

Describe the tubuloglomerular feedback mechanism?

A
  • Increased flow in tubule which increases NaCl to distal tubule
  • NaCl sensed by macula densa (part of JG apparatus)
  • Macula Densa -> vasoconstriction of afferent arteriole
28
Q

What is the urine concentration?

A

Ux mg/L

29
Q

\what is the urine flow rate?

A

l/min

30
Q

What are the Measured Variables?

A
  • Plasma Concentration (Px = mg/L)
  • Urine concentration (Ux = mg/L)
  • Urine flow rate (V = L/min)

These measured variables are used to get RPF, GFR, etc.

31
Q

What is the equation for Renal Clearance?

A

Cx = (Ux * V) / Px

Ux = Urine concentration 
V = Volume flow rate
Px = Plasma conc
32
Q

What are the units for Renal Clearence?

A

Litres/min (volume flow)

33
Q

What is the clearence of inulin the same as?

A

GFR

34
Q

Why does creatinine slightly overestimate GFR?

A

Approximate measurement

- Moderately secreted by renal tubules so slightly overestimates

35
Q

What formula can used to estimate GFR?

What is taken into account in the calculation?

A

Cockcroft-Gault formula

- Encorporates age, weight, gender and Cr level to estimate GFR

36
Q

What substance can be used to estimate Renal Plasma Flow (RPF)?

A

Para-aminohippuric acid (PAH)

  • Clearence of substance = Plasma to kidney
  • 100% of PAH that enters kidney leaves blood in urine
  • If it is not filtered through the glomerulus then it will be secreted by the nephron
37
Q

Why is the true renal plasma flow slightly higher than that estimated by PAH?

A

Not all blood that goes to the kidneys is filtered - goes to other areas of kidney (underesteimates by ~ 10%)

38
Q

When are NSAIDs avoided?

A

In patients with HF and Renal Failure

39
Q

What does it mean if the amount filtered (filtered load) is the same as the amount of a substance excreted?

A

No secretion/resorption

40
Q

What does it mean if GFR vis greater than nclearence?

A

Resorption must be occurring

41
Q

What does it mean if GFR

A

Secretion must be occurring

42
Q

What is the equation to work out Renal blood flow (RBF) from renal plasma flow?

A

RPF/(1-Hct)

Hct is usually ~ 40%
so usually RPF/0.6

43
Q

What is a roughly normal Filtration Fraction?

A

~ 20%

44
Q

What will the effect of prostaglandins be on RPF, GFR and FF?

A
  • Increase RPF
  • Increase GFR
  • FF will remain normal
45
Q

What will the effect of NSAIDs be on RPF, GFR and FF?

A
  • Decrease RPF
  • Decrease GFR
  • FF will remain normal
46
Q

What may be the clinical effects of NSAIDs on someone with bad kidneys?

A
  • Acute renal failure

- Acute HF (accumulation of Na+ and water)

47
Q

What are the effects of Angiotensin II on the RPF, GFR and FF?

A
  • Increases GFR
  • Decreases RPF
  • Increases FF
48
Q

What are the effects of ACEi on RPF, GFR and FF?

A
  • Decreased GFR
  • Increased RPF
  • Decreased FF
49
Q

What is the equation for calculating the amount of substance x being excreted?

A

Excreted (X) = V*Ux

50
Q

What is the equation for calculating the amount of substance x being filtered?

A

GFR*Px

51
Q

The amount excreted equals what?

A

Filtered - Reabsorbed + Secreted

52
Q

What are examples of regulated solutes? (or solutes which do not have a concentration change in renal failure)

A
  • Na+ (by ADH)

- K+ (by aldosterone)

53
Q

What are the unregulated solutes in renal failure?

A
  • Creatinine

- Urea

54
Q

What is the filtered load?

A

Amount of substance x that is filtered into Bowmans space per unit time
- Same as amount filtered

55
Q

What happens to the filtered load of K+ and Na+ in renal failure?

A

Decreased

56
Q

What happens to the levels of Na+, K+ and Urea that are excreted in renal failure?

A

The levels of the solute that are excreted remain the same as in normal kidney function
- The amount excreted must = the amount taken in/ produced by body

57
Q

What is fractional excretion?

A

Excretion / Filtered load

58
Q

What happens to the fractional excretion of Na+ and K+ in renal failure?

A

Increased (excreting much more of what they are filtering, although are filtering less)

59
Q

What happens to the fractional excretion of Urea in renal failure?

A

Remains the same

60
Q

What is the clearence equation?

A

C = (U x V) / (P)

C = Clearence
U = Urine conc.
V = Urine flow rate 
P = Plasma conc.

Clearence of inulin = GFR

61
Q

What is larger Renal Blood flow or renal plasma flow?

A

Renal Blood flow

RPF/1-Hct

62
Q

What is the equation to work out the amount filtered across the glomerulus (filtered load)?

A

GFR * Px

63
Q

What is the equation for working out the amount of a substance excreted?

A

Excreted = Filtered - Reabsorbed + Secreted

Filtered = GFR * Plasma conc

USMLE Renal (11 decks)

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